W IOS od wersji 12.4 istneje możliwość stosowania technologii
Reliable Static Routing.
Celem jest przy wykorzystaniu 2 ISP stworzenie tras zapasowych bez wykorzystania BGP.
Na tej podstawie sprawdzamy dostępność routera "next hope" lub innego docelowego.
W momencie jego niedostępności router przełącza nas na drugą domyślną ścieżkę.
Technologia opisana jest dokładniej na stronie: http://www.nil.com/ipcorner/SmallSiteMultiHoming/
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tekst źródłowy
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Small Site Multi-homing
by Ivan Pepelnjak, Damijan Markovič and Dragan Spasojevič
Unless your network operates under extreme security considerations or in places where the is no Internet access, your management has probably already asked you to lower the wide area network (WAN) costs by migrating from a traditional leased line or frame-relay-based network to an Internet- or MPLS VPN-based transport, while at the same time retaining or even increasing network reliability. This conflicting set of requirements might force you to make all your sites multi-homed (connected to more than one Internet Service Provider – ISP).
Multi-homing requirements aren’t new; for example, every decent e-commerce solution should be multi-homed. However, most solutions you’ll find with extensive help from Google require that you possess your own public IP prefix and an autonomous system number (both of them are scarce resources) and run Border Gateway Protocol (BGP) with all ISPs. Clearly, these requirements are completely unrealistic if you want to multi-home a small remote office.
In this article, you’ll learn how to:
connect a small remote site to more than one ISP;
detect failures in the ISP networks and adjust your routing accordingly;
increase overall availability of your sites with Service Level Agreement (SLA) monitoring;
log all relevant changes in the remote site connectivity.
Basic Small Site Multi-Homing
Connecting a small site to multiple service providers can be extremely easy – you get two upstream links and two provider-assigned (PA) IP addresses (either static or dynamically assigned). Since each ISP will give you only a single IP address, you have to use private IP addresses on the LAN side of the router (Figure 1).
Figure 1
IP addressing in small multi-homed site
As most ISPs will not be willing to run a dynamic routing protocol with small sites, you have to configure static default routing on your end. You would almost always prefer one provider over the other, resulting in a primary and a backup default route (Figure 2).
Note
With careful configuration, it’s also possible to achieve rudimentary load sharing with two equally-good default routes.
Figure 2
Static default routing
The router on the remote site would also have to perform two independent NAT translations, one for packets sent to ISP A (where local addresses get translated to the IP address assigned by ISP A) and another one for packets sent to ISP B (Figure 3).
Figure 3
NAT translation in small multi-homed site
One of the major issues in multi-homed site design is the proper handling of the return traffic. It’s not uncommon to experience performance problems if the outbound and return traffic flow over different links (also known as asymmetrical routing), while IP multicast and stateful packet inspection (part of IOS firewall feature set) almost always break under these conditions. Fortunately, asymmetrical routing is never a problem in a dual NAT design from Figure 3, as the source address of the outbound packet indicates the link that has been used to send it (see Figure 4).
Figure 4
Symmetrical routing with dual NAT
Configuring Small Multi-Homed Site
Configuring the gateway router in a small multi-homed site is very simple. You start by configuring the private and public IP addresses (Listing 1).
Listing 1
Initial router configuration
hostname GW
!
ip cef
!
ip dhcp pool LAN
network 192.168.0.0 255.255.255.0
default-router 192.168.0.1
!
interface FastEthernet0/0
description *** Inside LAN interface ***
ip address 192.168.0.1 255.255.255.0
!
interface Serial0/0/0
description *** Link to ISP 1 ***
ip address 172.16.1.1 255.255.255.252
!
interface Serial0/0/1 point-to-point
description *** Link to ISP 2 ***
ip address 172.17.3.1 255.255.255.252
NAT configuration is a bit more complex; you have to configure two NAT pools (one for each ISP), as displayed in Listing 2.
Listing 2
Network Address Translation configuration
interface FastEthernet0/0
ip nat inside
!
interface Serial0/0/0
ip nat outside
!
interface Serial0/0/1 point-to-point
ip nat outside
!
ip nat inside source route-map ISP_A interface Serial0/0/0 overload
ip nat inside source route-map ISP B interface Serial0/0/1 overload
!
route-map ISP_A permit 10
match interface Serial0/0/0
!
route-map ISP_B permit 10
match interface Serial0/0/1
Note
Having two route-maps matching outgoing interfaces (the match interface statement in a NAT route-map matches outgoing interface) is the only way to configure per-interface NAT pools in Cisco IOS.
As most ISPs will not be willing to run a dynamic routing protocol with small sites, you have to configure static default routing on your end. You would almost always prefer one provider over the other (therefore one default route would have a lower administrative distance) as shown in Listing 3, although it’s possible (with CEF switching using per-destination load sharing) to use two default routes in 1-to-1 load-balancing setup.
Listing 3
Basic multihomed default routing setup
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 10
ip route 0.0.0.0 0.0.0.0 Serial0/0/1 251
The simplistic static routing in Listing 3 represents a major availability issue – if you cannot detect the link failure on the link toward ISP A reliably, the default static route toward ISP B will never be used. While you can almost always detect leased-line or cable failure (due to loss of carrier signal) and usually detect Frame-Relay failures through Local Management Interface (LMI) messages or end-to-end keepalives, it’s almost impossible to detect layer-2 failures in PPPoE (ADSL) or Metro Ethernet access layers. In these scenarios, the primary default route will never disappear (even though the next-hop router is no longer reachable), making static multi-homing impossible. This problem is solved, however, in Cisco IOS release 12.3(8)T (integrated in release 12.4) with static routes tied to IP SLA measurements.
Not-so-Very-Static Routes
Cisco IOS release 12.3(4)T introduced Enhanced Object Tracking, which together with Reliable Static Routing Using Object Tracking introduced in IOS release 12.3(8)T solves the problem. Enhanced Object Tracking introduces a generic track object that can track a state of an interface (layer-2 or layer-3 state), presence or metric of an IP route, state of an SLA measurement or even availability of Mobile IP home agent or GPRS nodes. Even more, you can combine various track objects (including weighing them) into a compound object.
The Reliable Static Routing Using Object Tracking feature ties a track object to a static route – whenever the track object’s state is down, the static route is removed from the routing table; exactly what you would need to support reliable multi-homing. To configure a static route based on the state of the next-hop router, you need to:
Configure an ip sla (previously known as Response Time Reporter – rtr) object pinging the next-hop router on primary Internet link (Listing 4). The polling frequency you specify (in seconds) depends on the reliability requirements, but anything below a few seconds would place unnecessary burden on the next-hop router (as you might not be the only one tracking its availability).
Listing 4
Pinging next-hop router
ip sla 100
icmp-echo 172.16.1.2 source-interface Serial0/0/0
timeout 500
frequency 3
ip sla schedule 100 life forever start-time now
Note
You cannot change the parameters of an SLA object once you’ve scheduled it. To change the target IP address, timeouts or polling frequency, you need to delete the SLA object and recreate it.
Create a track object monitoring the reachability of the SLA target (Listing 5). As you probably don’t want to respond to a single lost ICMP packet, you should use the delay option of the track object to specify how long the next-hop router should remain unreachable before it’s declared to be lost (the down delay should be approximately three times the SLA polling frequency and the up delay should be even longer).
Note
When calculating the up delay, remember that a router can temporarily respond to pings during the bootstrap process.
Listing 5
Tracking the state of the next-hop router
track 100 rtr 100 reachability
delay down 10 up 20
After configuring the track object, attach it to the primary static default route to ensure that the default route is removed if the next-hop router is not reachable (Listing 6).
Listing 6
Conditional static default route
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 10 track 100
ip route 0.0.0.0 0.0.0.0 Serial0/0/1 251
You can check the proper operation of the reliable static routing with the show ip route command. Listing 7 displays the IP routing table on the GW router when the primary next-hop router is available, Listing 8 shows the routing table after primary next-hop router failure.
Listing 7
IP routing table with operational primary next-hop router
GW#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.17.0.0 255.255.255.252 is subnetted, 1 subnets
C 172.17.3.0 is directly connected, Serial0/0/1
172.16.0.0 255.255.255.252 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial0/0/0
C 192.168.0.0 255.255.255.0 is directly connected, FastEthernet0/0
S* 0.0.0.0 0.0.0.0 is directly connected, Serial0/0/0
Listing 8
IP routing table after the next-hop router failure
GW#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.17.0.0 255.255.255.252 is subnetted, 1 subnets
C 172.17.3.0 is directly connected, Serial0/0/1
172.16.0.0 255.255.255.252 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial0/0/0
C 192.168.0.0 255.255.255.0 is directly connected, FastEthernet0/0
S* 0.0.0.0 0.0.0.0 is directly connected, Serial0/0/1
Monitoring Reliable Static Routing
The reliable static routes silently appear or disappear from the IP routing table based on the state of the attached track object; the only means of monitoring their state is with the show ip route track-table command (Listing 9) or with the debug track command (Listing 10).
Listing 9
Show tracked routes
GW#show ip route track-table
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 10 name ISP_A track 100 state is [down]
Listing 10
Debug tracking
GW#debug track
06:49:44: Track: 100 Down change delayed for 10 secs
06:49:54: Track: 100 Down change delay expired
06:49:54: Track: 100 Change #26 rtr 100, reachability Up->Down
06:50:24: Track: 100 Up change delayed for 20 secs
06:50:34: Track: 100 Up change delay cancelled
06:58:59: Track: 100 Up change delayed for 20 secs
06:59:19: Track: 100 Up change delay expired
06:59:19: Track: 100 Change #25 rtr 100, reachability Down->Up
Note
The debugging printout in Listing 10 illustrates a real-life scenario where the next-hop router became temporarily reachable during the bootstrap process and disappeared a few seconds later (the change delay cancelled printout).
While the silent modification of the IP routing table might be acceptable in most situations (after all, you don’t get notified when a regular IP route disappears from the routing table either), you might want to know if your primary ISP is unreachable (similar to the interface up/down events you would get with traditional access methods like leased lines or Frame Relay access). The Embedded Event Manager 2.2 (introduced in IOS release 12.4(2)T) is the ideal solution, as you can trigger EEM applets (or TCL scripts) whenever a track object’s state changes with the event track configuration command.
To display the changes in a tracked object state, you can define two EEM applets, one triggered on the down change, another one triggered on the up change. If you only want to be notified that the state has changed, the only action you need to specify is the syslog msg action, but you can perform any number of actions you want (for example, send an e-mail to the network manager or even reconfigure the router). A sample EEM configuration is shown in Listing 11 and the printouts generated by it are included in Listing 12.
Listing 11
IOS EEM generates syslog messages on tracked object state change
event manager applet ISP_A_down
event track 100 state down
action 1.0 syslog msg "ping to 172.16.1.2 from Serial 0/0/0 failed"
event manager applet ISP_A_up
event track 100 state up
action 1.0 syslog msg "172.16.1.2 is reachable"
Listing 12
Sample EEM printouts
07:02:19: %HA_EM-6-LOG: ISP_A_down: ping to 172.16.1.2 from Serial 0/0/0 failed
07:03:19: %HA_EM-6-LOG: ISP_A_up: 172.16.1.1 is reachable
End-to-End Connectivity Test
After you’ve successfully implemented the tracking of the primary next-hop router’s availability, you might be tempted to improve the solution to track end-to-end connectivity through ISP A and switch to the backup ISP whenever your central site is not reachable through the primary ISP. In theory, the required configuration change should be minimal – you only have to change the destination IP address in the IP SLA definition (Listing 13).
Listing 13
Pinging a remote host
hostname GW
!
ip sla 100
icmp-echo 172.29.0.1 source-interface Serial0/0/0
timeout 200
frequency 10
ip sla schedule 100 life forever start-time now
In most cases, that’s all you have to do. As the ICMP echoes sent to the central site come from an IP address belonging to ISP A (the IP address configured on Serial 0/0/0 in the example), it’s highly unlikely that you would get a return packet if the ISP A has problems. However, the return packet might still reach your router under rare circumstances (misconfigured access lists or one-way connectivity in ISP A). The results are astonishing:
As the pings through ISP A (primary default route) fail, the router removes the primary default route and the backup default route through ISP B is installed.
Pings are now sent from an IP address belonging to ISP A on a path going through ISP B.
If there is a return path from the central site to the IP address sending the ICMP packets, the central site will yet again appear reachable and the primary default route will be reinstalled (resulting in connectivity loss).
Due to renewed connectivity loss, the router will oscillate between the two default routes (Listing 14).
Listing 14
Oscillating routing
GW#debug track
07:15:09: Track: 100 Change #32 rtr 100, reachability Up->Down
07:15:09: %HA_EM-6-LOG: ISP_1_down: ping to 172.29.0.1 from Serial 0/0/0 failed
07:15:19: Track: 100 Up change delayed for 20 secs
07:15:39: Track: 100 Up change delay expired
07:15:39: Track: 100 Change #33 rtr 100, reachability Down->Up
07:15:39: %HA_EM-6-LOG: ISP_1_up: 172.29.0.1 is reachable
07:15:49: Track: 100 Change #34 rtr 100, reachability Up->Down
07:15:49: %HA_EM-6-LOG: ISP_1_down: ping to 172.29.0.1 from Serial 0/0/0 failed
07:15:59: Track: 100 Up change delayed for 20 secs
To fix this (admittedly rare) problem you have to configure a local policy routing (as the ip sla packets originate within the router, they are only affected by the ip local policy) that matches ICMP packets being sent from the Serial0/0/0 interface (based on their IP address; the PingISP_A access list) and forces them to be sent out through the same interface with the set interface configuration command (Listing 15).
Listing 15
Fix the oscillating routing with local policy
ip local policy route-map LocalPolicy
!
ip access-list extended PingISP_A
permit icmp host 172.16.1.1 host 172.29.0.1
!
route-map LocalPolicy permit 10
match ip address PingISP_A
set interface Serial0/0/0
If you want to, you can extend the concepts presented in this section even further. For example, if the central site is not reachable through either ISP (it might be down), it could make more sense to retain ISP A as the primary ISP. You would thus need to track the central site’s availability through both ISPs and configure a reliable static default route for both of them (the backup one with a higher administrative distance, of course) with a third (last-resort) default route pointing to ISP A. The complete configuration is included in Listing 16 and its interpretation is left as an exercise for the reader.
Listing 16
GW router tracking central site availability through both ISPs
hostname GW
!
ip cef
!
ip dhcp pool LAN
network 192.168.0.0 255.255.255.0
default-router 192.168.0.1
!
ip sla 100
icmp-echo 172.29.0.1 source-interface Serial0/0/0
timeout 200
frequency 3
ip sla schedule 100 life forever start-time now
!
ip sla 101
icmp-echo 172.29.0.1 source-interface Serial0/0/1
timeout 500
frequency 3
ip sla schedule 101 life forever start-time now
!
track 100 rtr 100 reachability
delay down 10 up 20
!
track 101 rtr 101 reachability
delay down 10 up 20
!
interface FastEthernet0/0
ip address 192.168.0.1 255.255.255.0
ip nat inside
!
interface Serial0/0/0
description *** Link to ISP 1 ***
ip address 172.16.1.1 255.255.255.252
ip nat outside
!
interface Serial0/0/1
description *** Link to ISP 2 ***
ip address 172.17.3.1 255.255.255.252
ip nat outside
!
ip local policy route-map LocalPolicy
!
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 10 track 100
ip route 0.0.0.0 0.0.0.0 Serial0/0/1 11 track 101
ip route 0.0.0.0 0.0.0.0 Serial0/0/0 250
ip route 0.0.0.0 0.0.0.0 Serial0/0/1 251
!
!
ip nat inside source route-map ISP_A interface Serial0/0/0 overload
ip nat inside source route-map ISP B interface Serial0/0/1 overload
!
ip access-list extended PingISP_A
permit icmp host 172.16.1.1 host 172.29.0.1
ip access-list extended PingISP_B
permit icmp host 172.17.3.1 host 172.29.0.1
!
route-map ISP_A permit 10
match interface Serial0/0/0
!
route-map ISP_B permit 10
match interface Serial0/0/1
!
route-map LocalPolicy permit 10
match ip address PingISP_A
set interface Serial0/0/0
!
route-map LocalPolicy permit 20
match ip address PingISP_B
set interface Serial0/0/1
!
!
event manager applet ISP_A_down
event track 100 state down
action 1.0 syslog msg "ping to central site from Serial 0/0/0 failed"
event manager applet ISP_A_up
event track 100 state up
action 1.0 syslog msg "central site is reachable"
event manager applet ISP_B_down
event track 101 state down
action 1.0 syslog msg "ping to central site from Serial 0/0/1 failed"
event manager applet ISP_B_up
event track 101 state up
action 1.0 syslog msg "central site is reachable"
!
end
Summary
With the ever faster replacement of traditional WAN networks with MPLS VPN- or Internet-based solutions, it’s increasingly important to have a good design and implementation strategy for small multi-homed sites. While it’s easy to implement multi-homed sites whenever you are able to run a routing protocol between the customer edge (CE) and provider edge (PE) router, as is the case with most MPLS VPN implementations, the static default routing imposed on most Internet customers by their ISPs make reliable multi-homing almost impossible in modern networks that are not able to signal loss of layer-2 connectivity reliably.
The Reliable Static Routing Using Object Tracking feature available in Cisco IOS release 12.4 allows you to tie static route viability to a tracked object (interface, another route …). If you track the state of the next-hop router, it’s possible to detect layer-3 failures reliably, triggering a reroute to the backup ISP. You can improve this design, track the end-to-end availability of the central site and reroute to the backup ISP whenever you cannot reach the central site through the primary ISP. Even more, you don’t have to rely on ICMP echo packets; IP SLA feature of Cisco IOS can track availability of a large number of applications (for example, your company’s central web server).
