# 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 ## Answer ``` #R1-R3 router isis mpls ldp autoconfig ! ip prefix-list HOST_ROUTES permit 0.0.0.0/0 ge 32 le 32 ! mpls ldp label allocate global prefix-list HOST_ROUTES #R4-R6 router isis mpls ldp autoconfig ! mpls ldp label allocate global host-routes #XR1 router isis 1 add ipv4 mpls ldp auto-config ! mpls ldp address-family ipv4 label local allocate for HOST_ROUTES ! ipv4 access-list HOST_ROUTES permit ipv4 host 1.1.1.1 any permit ipv4 host 2.2.2.2 any permit ipv4 host 3.3.3.3 any permit ipv4 host 4.4.4.4 any permit ipv4 host 5.5.5.5 any permit ipv4 host 6.6.6.6 any permit ipv4 host 19.19.19.19 any permit ipv4 host 20.20.20.20 any #XR2 router isis 1 add ipv4 mpls ldp auto-config ! mpls ldp address-family ipv4 label local allocate for host-routes ``` ## 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. ``` #IOS-XE mpls ldp label allocate global prefix-list PREFIX_LIST #IOS-XR mpls ldp address-family ipv4 label local allocate for ACL ``` 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: ``` mpls ldp address-family ipv4 label local allocate for HOST_ROUTES ! ipv4 access-list HOST_ROUTES permit ipv4 host 1.1.1.1 any permit ipv4 host 2.2.2.2 any permit ipv4 host 3.3.3.3 any permit ipv4 host 4.4.4.4 any permit ipv4 host 5.5.5.5 any permit ipv4 host 6.6.6.6 any permit ipv4 host 19.19.19.19 any permit ipv4 host 20.20.20.20 any ``` 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. ``` #IOS-XE mpls ldp label allocate global host-routes #IOS-XR mpls ldp address-family ipv4 label local allocate for host-routes ``` ## 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.