Cisco 7600 Series Router Software Configuration Guide, Cisco IOS Release 15S

Node Failure

Node failure is handled through BFD protected static routes. If the primary node fails, BFD detects the failure and triggers a re-convergence of the network through the backup node. Since static routes are used on CSRs, the re-convergence only requires initialization of the secondary or backup route.

Figure 1-2 explains a node failure in a network with BFD over SVI on a REP enabled ring topology. The routers 7609-1, 7609-2 and the two 2941 routers in the network are protected nodes. If there is a failure on the primary node (7609-1), traffic is redirected to the secondary node (7609-2). The Layer 2 nodes that are numbered from 1 to 8 are not protected.

Figure 1-2 Node Failure

When the primary node encounters a failure, the following events take place:

1. At Layer 2 level, REP detects that the primary path is faulty and opens the secondary path.

2. At Layer 3 level, BFD detects the primary node failure and trigger network convergence to the secondary node.

Link Failure

Link failure in the Layer 2 network is handled by REP. In most cases, REP detects a link failure in less than 100 milliseconds in the ring. In case the primary Layer 2 path fails, REP opens the secondary Layer 2 path for Layer 3 traffic.

Figure 1-3 explains link failure in a network with BFD over SVI. If there is a primary link failure in the Layer 2 ring, REP detects at the Layer 2 level that the primary path is faulty and opens the secondary path. As there is no node failure, BFD does not detect and trigger any network change.

Figure 1-3 Link Failure

REP should detect and act on any fault in the network before BFD. Otherwise, BFD may wrongly detect a node failure. If there is a link failure in the primary Layer 2 path and REP takes longer time than BFD to unblock the secondary path, BFD could wrongly detect a node failure. In that case, after unblocking the secondary path by REP, traffic flows through the secondary node.

For more information on configuring REP, see the following link:

http://www.cisco.com/en/US/docs/routers/7600/ios/15S/configuration/guide/rep.html

Summary Steps

1. enable

2. configure terminal

3. interface vlan number

4. ip address ip-address mask

5. bfd interval milliseconds min_rx milliseconds multiplier interval-multiplier

6. ip route static bfd vlan number gateway

7. ip route [vrf vrf-name] mask { ip-address | vlan vlan-number [ ip-address ]} [dhcp] [ distance ] [name next-hop-name ] [permanent | track number ] [tag tag ]

8. end

9. show ip static route

Detailed Steps

Note If you are downgrading the software version from 15.1(1)S to 15.0(1)S and you have BFD over SVI configuration, you should first run the platform bfd allow-svi command on the15.1(1)S software and then start the software downgrade.

The following example shows BFD over SVI configuration on routers A and B in the same VLAN and verifies the configuration. The show bfd neighbors details command is used to verify that BFD session is created.