ICMP packet type 5 - Redirect
+RVW$
,3
*:
5RXWHU%
+RVW&
,3
5RXWHU$
,QWHUIDFH
,3
YODQ
,QWHUIDFH
,3
YODQ
,QWHUIDFH
,3
YODQ
,QWHUIDFH
,3
YODQ
6XEQHW$YODQ
6XEQHW%YODQ
6XEQHW&YODQ
,QGXVWULDO(WKHUQHW
Host A wants to send an IP frame to host C. Host C is not located in the same subnet as host
A. For this reason host A sends the IP frame to its default gateway. The default gateway of host
A is interface 1 of router A. Router A cannot forward the IP frame because it does not know the
destination network. Via its routing table, however, router A knows that subnet C is reachable
via router B. Router B connects subnet A with subnet C. Router A sends a redirect message to
host A. In this, router A instructs host A in future to send IP frames to host C via router B whose
IP address is contained in the redirect message. The initial IP frame is sent by router A directly
to router B that forwards it to Host C.
Conditions for sending redirect messages
● The IP frame is received and sent via the same interface of router A.
● The source IP address (host A) is from the same subnet as the next hop address (router B)
in the routing table.
● The IP frame is not affected by a source NAT rule (masquerading, source NAT or NETMAP).
● So that router A forwards the initial IP frame to router B, a firewall rule vlanX → vlanX is
required.
Technical basics
3.2 ICMP
SCALANCE S615 Web Based Management
34 Configuration Manual, 11/2019, C79000-G8976-C388-08
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