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LDP Local Allocation Filtering

Load isis.cfg

#IOS-XE
config replace flash:isis.cfg

#IOS-XR
configure
load bootflash:isis.cfg
commit replace
y

  • Configure LDP using IGP autoconfig on routers R1-R6 and XR1-2

  • On R1-R3 and XR1, allocate labels only for host routes using a prefix-list/ACL

  • On R4-R6 and XR2, allocate labels only for host routes without using a prefix-list or ACL

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Answer

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Explanation

By default, LDP will locally allocate labels for every non-BGP prefix in the RIB. However, generally LSPs are only formed on the loopbacks, so there is no reason to allocate labels for every transit prefix in the IGP. By allocating labels only for host routes, we can save some memory and better scale LDP.

To selectively allocate labels, we have two options. First, we can use a prefix-list/ACL to have granular control over what prefixes will be allocated a label.

The IOS-XR method is quite burdensome, because we cannot use a mask filter like we can with a prefix-list. This is the reason that we had to manually specify every host route in the ACL on XR1:

The second method is the built-in host-routes option. This instructs LDP to only allocate labels for /32 prefixes, and does not require any prefix-list or ACL.

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Verification

On any router in the topology, we should only see /32 prefixes in the LFIB and LIB. It does not matter which router we look at, because both the ACL/prefix-list method, and the built-in for host-routes method, have the exact same effect.

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